PURPOSE: To obtain an air vent filter excellent in air bubble removing capacity, by forming the title filter from a base plate having a liquid inflow passage, a liquid outflow passage and a liquid flow passage and a gas permeable and liquid impermeable filter element constituting the wall surface of the liquid flow passage and forming the fluid flow passage into a meandering shape.

CONSTITUTION: An air vent filter is constituted of a base plate 3 having a liquid inflow passage 1, a liquid outflow passage 2 and a liquid flow passage 5 and a filter element 4 constituting a part of the liquid flow passage 5. The liquid flow passage 5 has a flat meandering shape having an almost rectangular cross-section and respectively communicates with the liquid inflow and outflow passages 2 at both ends thereof. Further, the filter element 4 constitutes the single surface or both surfaces of the wide wall surface of the liquid flow passage 5 and can permit gas to permeate it but can not permit a liquid to permeate it. The chemical liquid from the liquid inflow passage 1 flows through the liquid flow passage 5 and the air bubbles contained in the chemical liquid are removed through the filter element 4 in this process. Since the liquid flow passage 5 has an almost rectangular flat shape and the wide wall surface thereof is constituted of the filter element 4, air bubble removing capacity is large.